Among steels being used in the oil sands industry, since the abrasion of the steel of a pipe being used in the transportation of an oil sand slurry in particular occurs due to sand particles having a diameter ranging from 200 μm to 300 μm and its replacement life span is about 1 year, a lot of cost and time are required for the purchase and replacement of steel piping.
Methods of mining oil sands may be broadly classified as an open-pit mining method and an in-situ recovery method, in which the application of a slurry pipe system is essential for the post-processing of oil sand ore in the open-pit mining method. Crushed oil sand ore that has been mixed with water may have the form of a slurry, may include about 35% of sand and about 500 ppm of salt, and may be transported at a speed ranging from 3.5 m/sec to 5.5 m/sec. During the transportation of the slurry, since sand particles may erode steel by moving along a lower end portion of an inner side of a pipe, pipe have been used in a manner in which they are rotated about 3 times a year in order to increase the effective service life of the steel from which they are made.
Also, corrosion due to salt as well as abrasion due to the moving sand may occur in the slurry pipe, and it is problematic that corrosion products formed by the result of the corrosion do not reduce a corrosion rate of the material, but are immediately removed by the moving sand. In particular, the erosion of the material may occur much faster in an environment in which corrosion and abrasion occur simultaneously, such as an operating environment of the oil sand slurry pipe, than an environment in which corrosion and abrasion occur separately.
There is a case in which a carbide coating treatment or a surface heat treatment is performed on the inside of the pipe in order to extend the lifespan of the pipe by delaying such erosion. However, since costs for such reprocessing process exceed replacement costs of the material, there is a need to develop a material having excellent resistance to the erosion caused by the slurry without the need for reprocessing.
In general, it is known that abrasion resistance of a material increases with an increase in hardness. However, since a pipe material must have strength and ductility suitable for pipe production in terms of characteristics thereof, it may be impossible to use high-hardness martensite for increasing the hardness of the material. Steels for an oil sand slurry pipe currently being used are American Petroleum Institute (API) grade line pipe steels, wherein thermo-mechanical control process (TMCP) ferritic steels are used, in which, in order to increase abrasion resistance of the material, strength is increased to a level able to allow a pipe to be commercially produced. Hereinafter, techniques currently being used for pipe steels having excellent abrasion resistance will be described.
First, Korean Patent Application Laid-Open Publication No. 1987-0010217 discloses a method of securing abrasion resistance by installing a ceramic plate in a steel pipe, and Korean Patent Application Laid-Open Publication No. 2000-0046429 discloses a method of manufacturing an abrasion resistant pipe by forming a hardfacing weld layer on an inner surface of the pipe using tungsten carbide or high-chromium powder.
However, both patents disclose techniques in which a surface of a typical pipe is reprocessed by using a high hardness material in order to secure abrasion resistance, wherein high costs are incurred due to the fact that reprocessing and long-term abrasion resistance may not be assured, because the reprocessed layer may be detached due to external impacts or defects therein.
Next, Korean Patent Application Laid-Open Publication No. 2001-0066189 discloses a method of securing abrasion resistance and impact toughness by performing a carburization treatment on a surface of low carbon steel. However, a pipe surface hardened by the carburization treatment may not only have limitations in a welding zone, but rapid abrasion of a matrix structure may also occur after the abrasion of the surface hardened layer.
Also, Korean Patent Application Laid-Open Publication No. 2007-0017409 discloses a method of manufacturing steels having high mechanical strength and abrasion resistance, and the steels provided by the above patent have compositions including 0.30 wt %≦carbon (C)≦1.42 wt %; 0.05 wt %≦silicon (Si)≦1.5 wt %; manganese (Mn)≦1.95 wt %; nickel (Ni)≦2.9 wt %; 1.1 wt %≦chromium (Cr)≦7.9 wt %; 0.61 wt %≦molybdenum (Mo)≦4.4 wt %; selectively vanadium (V)≦1.45 wt %, niobium (Nb)≦1.45 wt %, tantalum (Ta)≦1.45 wt %, and V+Nb/2+Ta/4≦1.45 wt %; less than 0.1 wt % of boron, 0.19 wt % of (sulfur (S)+selenium (Se)/2+tellurium (Te)/4), 0.01 wt % of calcium, 0.5 wt % of a rare earth metal, 1 wt % of aluminum, and 1 wt % of copper; and iron as well as other unavoidable impurities as a remainder.
However, since the steels of the above invention contain carbon in an amount equal to or greater than that included in a medium carbon steel and large amounts of Ni, Cr, Mo, Nb, or V are used as alloying elements, manufacturing costs may not only be significantly increased, but mechanical strength may also be high. Therefore, the steels may not be suitable for a pipe material.
As another related art invention, Korean Patent Application Laid-Open Publication No. 2000-0041284 provides a method of manufacturing tool steels by spray forming, in which a method of increasing toughness by refining a size of carbide using Mo is disclosed. However, since manufacturing costs and strength may be high similar to the steel of Korean Patent Application Laid-Open Publication No. 2007-0017409, there may be limitations in using the steels as pipe materials.
Furthermore, Korean Patent Application Laid-Open Publication No. 2004-0059177 provides a method of manufacturing a steel having excellent abrasion resistance able to used for an oil pipe of a crude oil storage tank and piping in a ship's hull, wherein the steel according to the above patent is provided in such a manner that calcium (Ca)—Si in the form of a wire is added to a molten steel having a composition including 0.03 wt % to 0.1 wt % of C, 0.1 wt % to 0.3 wt % of Si, 0.05 wt % to 1.2 wt % of Mn, 0.05 wt % or less of phosphorous (P), 0.035 wt % or less of S, 0.03 wt % or less of aluminum (Al), 0.8 wt % to 1.1 wt % of Cr, 0.1 wt % to 0.3 wt % of copper (Cu), 0.1 wt % to 0.3 wt % of Ni, and iron (Fe) as well as other unavoidable impurities as a remainder, a degassing treatment is performed to control a Ca content to be in a range of 0.001 wt % to 0.004 wt %, and the steel is reheated to a temperature ranging from 1000° C. to 1200° C. and then hot-rolled at a temperature above Ar3.
The above invention improves abrasion resistance and corrosion resistance by improving density of a rust layer using Cr, Cu, Ni, and Ca. However, it may be impossible to secure abrasion resistance and corrosion resistance by using the rust layer in a severely abrasive environment such as that of an oil sand slurry pipe.
Therefore, demand for a steel sheet for an oil sand slurry pipe having good economic factors and production efficiency as well as excellent abrasion resistance and corrosion resistance, even in a severely abrasive and corrosive environment, such as an operating environment of an oil sand slurry pipe, has rapidly increased.